1. Technical Field of the Invention
The present invention relates in general to the telecommunications field and, in particular, to a method for emulating circuit-switched communications in a packet-switched transport network.
2. Description of Related Art
When introducing packet-based transmissions into former circuit-switched communications networks (e.g., typical wireless access networks), for a number of reasons it is important to be able to reuse the circuit-switched equipment where possible, and also meet the delay requirements of such a system. It is especially important to be able to reuse the already installed base equipment (e.g., in wireless access networks, the radio base stations and speech/data transcoders).
One of the driving forces behind the introduction of packet-based transport networks is the gain in transmission efficiency that can be utilized for discontinuous transmission (DTX) communications and statistical multiplexing applications. Consequently, it is important to be able to reuse installed base equipment and still obtain the highest gain in transmission efficiency possible.
There are a number of ways of converting a circuit-switched network for packet-switched communications, with the aim of reusing as much circuit-switched equipment as possible. One solution is to upgrade the software for the circuit-switched equipment so that it can handle packet-switched communications. However, in many cases, the circuit-switched hardware has inherent limitations that make such software upgrades virtually impossible to implement. Another solution is to use the method known as Circuit Emulation (CE). The CE concept builds on an approach whereby several circuit-switched channels are multiplexed into a packet which is transferred across a packet-switched network to an end-point or node. At this end-point, the received packet is de-multiplexed and the original circuit-switched bit stream is recreated.
For example, the above-described CE approach is used in Asynchronous Transfer Mode (ATM) communications technology with fixed length packets. The most significant drawbacks of this CE approach are that the resulting transmission efficiencies are lower than those experienced with the conventional circuit-switched solutions, and the different independent circuit-switched channels have to be unpacked before they can be switched within the packet-switched network. However, an advantage of the above-described CE approach is that it produces relatively good transmission delay characteristics.
The third solution that can be used to convert existing circuit-switched equipment for packet-switched communications is to insert a protocol converter between the circuit-switched and packet-switched networks. However, the transmission delay characteristics resulting from this gateway solution are typically not good enough for real-time services (e.g., speech).
Essentially, there are three issues to consider when a change in transport technology from circuit-switched to packet-switched technology is being planned: delay; transport efficiency; and equipment reuse. The delay associated with the transport technology is an important issue to consider when the payload involves real-time services (e.g., speech). As such, the existing CE approaches can solve this real-time service problem, but not in a transport efficient manner. The transport efficiency of the technology to be used is another important issue to consider in certain applications, such as in cellular access networks. However, transport efficiency is not a major issue when bandwidth is inexpensive, but in many applications this is not the case. In any event, the basic problem that needs to be addressed is how to change a transport technology in a mobile communications system from circuit-switched to packet-switched so that an already installed circuit-switched infrastructure can be maintained, and an efficient transport with a relatively low delay can still be obtained. As described in detail below, the present invention successfully resolves this problem and other related problems.
In accordance with a preferred embodiment of the present invention, a circuit emulation method with garbage removal is used to convert a circuit-switched network for packet-switched transmissions while reusing as much of the existing circuit-switched infrastructure as possible. The garbage removal function removes unnecessary information before it can be transported, which increases the transport efficiency and decreases the transport delay.
An important technical advantage of the present invention is that circuit-switched equipment can be used in a packet-switched transport network, without making changes to the circuit-switched equipment regardless of existing hardware limitations.
Another important technical advantage of the present invention is that existing circuit-switched equipment can be reused for packet-switched transmissions, while fulfilling the delay requirements placed on real-time services and not wasting transmission resources.
Still another important technical advantage of the present invention is that circuit-switched equipment can be used in a packet-switched transport network in a manner that increases the transport efficiency and decreases the transport delay.